It depends quite a bit on party composition too. A party with a lot of healing, but low damage output, needs to rest more often. A party with a lot of temporary hit points and high damage output needs to rest less often.
At low levels, defenders seem to go through 2-3 surges each combat. If they have 11 to start with, thats 4-5 combat encounters worth. If the party is ambushed, that number can quickly go down to 2-3 encounters, because when enemies land some lucky hits on the Rogue or Wizard, it's a pretty huge resource drain.
At higher levels, there is more healing help that doesn't necessarily involve triggering healing surges, and more encounter powers to use, so you don't need the dailies as quickly, and 6-7 encounters are easily doable, depending on the difficulty of the encounters.